Attenuation regulator



Nov. 7, 1944, s. DARLINGTON ATTENUATI ON REGULATOR Filed on. '7, 1942 4 Sheets-Sheet 1 FIG. 2

commvrn CONSTANT-R NETWORK NETWORK FIG. 6'

Fla. 3

a a a R causnwr-n NEIWORK IN [/5 N TOR S. DARL/ 6 TON A 7 TORNEK NOV. 7, 1944. RU GT N I 2,362,359

ATTENUATION REGULATOR Filed Oct. 7, 1942 4 Sheets-Sheet 2 FIG. 7

. lfi l I 2" a 4 I R}! 5 ll l'A'l'lv I v I 3 a5; I T 4 INVENTOR- 5. DARLING TON ATTOP/VEV Nov. 7, 1944.

S. DARLINGTON ATTENUATION REGULATOR Filed. Oct. 7, 1942 4 Sheets-Sheet 5 IN VEN TOR ATTORNQ NOV. 7, 1944. DVARLINGVTONV 2,362,359

-ATTE'NUATION REGULATOR Filed Oct. 7, 1942 4 Sheets-Sheet 4 FIG. /5

AAAA MM Vvn 5. DARLYNGTON A TTOR/VEV Patented Nov. 7, 1944 2,362,359 r y a ATTENUATION REGULATOR.

Sidney Darlington, v1*Tevv,-Yorli,, N. Y.,. assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation or New York' Application octoberi'l, 1942-, Serial: No. 461,171 5 Claims. (01. 178-44) This invention relates to attenuation equalizers and more particularly to variable equalizers having a plurality of independent controls.

The principal objects of the invention are the reduction of interaction between the independent controls and the reduction of the constant attenuation of the equalizers.

In long distance transmission: systems,.1-such,- for example, as multiplex carrier telephone systems, the. maintenance of uniform transmission levels at all times requires the use of automatically controlled attenuation; regulators tocompensate for the effects of varying temperatures upon the attenuation in the transmission line. These regulators are. usually located at several intermediate'repeater points in the system and are controlled by pilot currents of suitable frequencies, the amplitudes of which refiect' the variations of the line attenuation. The change in the attenuation varies in a rather complex manner with frequency in any given frequency range, but is such that it can be substantially compensated by the addition toor the subtraction from the attenuation characteristic 'at a selected normal temperature of fractional parts;

of a definite frequency dependent attenuation. The compensation may be-effectedby means of a single regulator of any of the various types disclosed in United States Patent 2,096,027, issued October 19, 1937, to H. W. Bode, but for variousreasons it has been found advantageous to employ a plurality of regulators, each adapted to control a simple component of the attenuation variation. Thus, the attenuation variation may be broken down into components representing respectively fiat loss, slope, and bulge frequency" characteristics or, in other words, components of the single control type-are: particularly sub- -ject to interaction effects when closely coupled in tandem, since such networks cannot be con-- structed to have constant i'mageimpedances.

Heretofore, the difficulty has been overcome by .tem.

The present invention prov-ides regulators of a new type in the form of unitary passive networksv having. two. variable im pedances which control the attenuation independently of each other, in difierent desired manners and which theother mayv control the attenuationindependently in accordance with a desired bulge characteristic or in any. other. assigned. manner. An

' important characteristic of the regulation is that varying according to the zero,first, and higher powers of frequency, and each component may be separately regulated. This. general method of I regulation is disclosed in United States Patent 2,246,307, issued June 17,. 1941,-t0. H. K. Krist.

- be kept at a negligibly small value if; accuracy of compensation is to be maintained. Equalizer variations of the control..impedances by equal percentage amounts inopposite directions from a normal. value add and. subtractequal fractions of adeflnite frequencydependent attenuation.

The nature of the inventionwill be more fully understood from the following'detailed description andbyreferences tothe attached drawings of'w-hi'ch:

Figs. 1 to-4iare schem'aticsxused to illustrate thetheory-of the invention;

Fig. 5 shows the circuit configuration of one form of regulator in accordance with the 'invention; I

Fig. 6 illustrates certain principles used in the design ofv regulators according to the invention;'

cording to the invention.

Mathematicalwtheory For'the purposeof developing the principles underlying the invention. it is. convenient to treat put source. oi resistance R r isconnected to the pair.ofterminalsfitaf and a load resistance Rg2 is connected to terminals 4,4. The variable imshown at Z1 and Z2 connected respectively to terminals l, l' and 2, 2'. In general, the impedances may be unrestricted in character, but the use of resistance elements for all of the impedances except Z1 and Z2 results in greatly simplified design and construction. The source and load impedances can generally be made resistive or substantiallyso.

The characteristic of the system with which the problem of regulation is primarily concerned is the change in the load current or voltage as the impedances Z1 and Z2 arevaried. It is necessary that the changes produced by the two variable impedances should be substantially findependent and it is desirable that their magnitudes should vary in accordance with a particular mathematical law which will be described'later'. The way in which these results are obtained by the networks of the invention will be explained in the'following mathematical treatment.

Let'it be assumed that the impedances Z1 and Z2 have normal values Z01 and Z02 about which the variations take place and let It may then be shown that the most typical form of the expression for the insertion loss between the source Rgl and the load Rgz is inwhich case 00 represents the insertion loss of the network when x is unity. This relationship has the property that replacing a: by its reciprocal replaces the loss variation e ""'o by its reciprocal. In other words, replacing :r by its reciprocal replaces the insertion loss characteristic by its image with respect to the normal loss characteristic 00. In the case'of the two-control regulators of the invention,'the-networks are so designed that the same transformation is effected when both :01 and x2 are simultaneously replaced by their reciprocals. This result is achieved by designing the circuit so that the following equalities are obtained between the a and b coeflicients of the insertion loss expression:

Under these conditions the insertion loss expression, Equation 1; becomes from which it is readily seen that replacing an and x2 simultaneously bytheir reciprocals produces a reciprocal value of e ""0).

The regulators according to the invention hav the further property that if either variable impedance is kept fixed at its reference or normal value, the variation of the insertion loss'as the other impedance is varied takes place in accordpedances by which the regulation is effected are ance with Equation 2. Thus, if :82 be made equal to unity, Equation 4 becomes 1+ 0 1 e -ea M 1+ n +x 1 [12 l and if an be made equal to unity,

"Except for a small interaction effect which is absent under the conditions specified above,

( Equations Band 6 define the loss contributions due to each of the separate controls regardless of the adjustment of the other. In other words, these equations define the principal parts of the loss contributions under any conditions of adjustment, the total loss being substantially equal to the sum of the two losses so defined. To demonstrate this, it is convenient to transform Equation 4 as follows:

Since g an y e" 1 Equation 4 may be transformed to v tanvh o 0) 1 2) (111 2)( 1 2) (8) 2 n)( 1 2)+( 1+ 2)( i+:v2)

which may be shown to be the same as wherein the quantities G1, G2 and K have the values 1 tanh 2 (0-002 defined by Equations 5 and 6 be denoted by I 1.and 1:2 respectively, it may be shown with the helpof Equation 7 that a $2 1+K tanh tanh 2 Equation 12 shows that so long as the iactorK is very nearly equal to unity tanh =tanh approximately (13) that is, that the total attenuation change is approximately equal to the sum of the two independently controlled components defined by Equations 5mm 6.

It is evident from Equation 12 that if K could i marinade; exactly i :to lmitma Equation would define an:exactiinsteadaotlamapproximate relationship. This would appear at first sight toprovide an ideal 'solutiomwhich would permit t'l iedesign of two-control equalizers with zero interaction between the controls. It turnsput, however, that this condition cannot be met in a passive system without the normal loss 00 becoining infinite or, alternatively, without one or the other of the loss Components in and I z becoming: zero. Inapracticerit hasbeen found that akdeparture of the value; of;K'jrome'untty'gbycas little as '2= pier cent results 'quitezsma-ll normal" less andthat. departures asavgreatv as :10: per; cent as not give .rise to excessive interactionwefiectsv- 1 It: maylbe" shown that: the physical condition corresponding to unity avalue of Kris that the two branches containingthe variableyimpedan'ces should bev-coniugatexorl-ifi other WQrds',-,that athe transfer-z'admittancelzbetween the t-two independeelite? meshes including the impedance, should zerio. 1T0 avoid the conjugate relationship 113715 necessary; that the v coefiicient-rm infillquation;- 4 should not be equal to thezproducti (11(12 bru-t, in

orderizthatzKmay becloseytm mitMsitisdesirableathat the. difierence between these-1. quantities should besmallrelatively to aim. e

' ltlhasalready been pointed-out that-it is? ad;

vantageous Y to ;1188- l only; pure resistances-{in t the; conplmsrsystem, theeoeffleients ou ta; and wand. therzrelatedfeet rs G1; 2 an th l -i jt simple-numbers; In that case thQyflfiQl-IBHQY deg: p ndenc'e I at the regulator lcharacteristiespisid terminedlwho y yt echan i fi h of 1 he ar a e; impedances ;Z1 and Z2, Burt hen s imp'lific'atiorigoi the design is achieved by constructing-.15 thqyarjieble; 'impedances imthe-zfor-tn sh tin Fig" 2- This figure showsamodined form? he \gai 'iable impedance Z1 comprising a four: mini-com] stant resistance network N1 terminated at one" pair of terminals"'lz'vy awariable resistance R1 and adapted to be conne cted at the other pair of tern'etwork .N'i" maybe o'f anyofth stant resistan e types; such, for example, described by O. J: Zoteiin the nrealJourna1;;Ju1y 19-2ai in: Distortion correctioniin electrical" constant resistance recurrentnetworksri i hse" networks are characterized equal constant re:- sistanc'e' image impecfances at their twopairs" or tennatioris ofany fihysi'c'all'y"realizable fredg' envy atte dan e. f l it the ima e impedance of thenetwork 1 denoted by'nmf and its" trans-fei constant By -1 their the-imp dance measuredat terminate-1,; if

- has'thevalue Whenthe variable -resistance..R)1 takesl the Value Roma-the, impedance Zi hecomes, equal ,tofwfitii which;v accordingly, may btt'ake'n'as the reiernce oz normal alue-ofUZi transflonneato where- 1 and-fiomithismayfbe drived ffefeiatitasiira J l ll 8121a,.

Equation 14; may

v es-ameype oflconsftruction-isufsed for the; econd-. variabl impedance; Zea; similar ex,-

pre's'smxiwni est ment:

namely;

substitutiiigtne'se} alu's ofe iande 'iii nqifation waivesthe value or the totalatt'enuationvaria tion with-substantiaI-accuracyas N tw'dzle: design Referring to Equations 17 and 18,,it will be seen,that the factors G1 and G1 determine the zitent to'rwhich th transfer constants 41211 and' ipfi of th'e' constant resistance?- networks enter into the" o'ver-allscharacteristics of the regulatord: Tll'i'e' valueskof these factors may therefore be arrived at from a consideration of the desired rfg fl-atzl'cir'i characteristics" and the". characteristics of- *awailable -constant resistance 'attfenu"alters. For :ithe

system'ttmberealizable physically the values must l-iet'between" the limits of plus-- andimim'ls unity; The factor-K whichvdetermines theinteraction effecfiamyn -he*cfroseniarbitrarilmi lficgnerah this should"beinearlyequal to unity, *but rit maxi-beneessaryetio adjustqthewalueby successive ztritcls toiavoldflextreme "orimpracti'cal valuesi of. certain otthe'; branch impedancess "Whether? the values at 1G1; and G2; should be positive vor negative and whether am sh'oultiebez greater or less than unity will depend uponthe type of configuration-adopt ed: for:- the complete. regulator circuit; The rules f governingthe choices will beagiven' "later.

1e; These values bf't'httlir with the basic network configurations of greater complexities are employed, additional arbitrary conditions may be assigned as required.

The design procedure along the lines indicated above is straightfQ Wfi d, but the computations are generally" lengthy and complex The use of resistive impeda'nces for the network branches and the use of the special formbf-vari'able impedance shown in Fig. 2, the reference or normal value of which is resistive,= greatly simplifies the design computations by making the a and D coeiiicients and the quantities G1, G2 and K all real numbers. It will be understood, however, that this is not an essential design condition and that other types of branch impedances may be used in accordance with the invention. Explicit design formulae, arrived at in the manner described, are given later fora number of representative networks. I

It has been pointed out in connection with Equations 5 and fi that the regulators of the invention provide variable insertion loss characteristics or the type expressed by Equation 2, namely,

when either of thevariable impedances is held fixed at its reference value and the other is varied. Single control regulators having the above characteristic are disclosedin the above-mentioned United States Patent 2,096,027 to H. W..Bode. Twobasic forms using variable impedances of the type shown in Fig. 2 are shown in Figs. 3 and 4, respectively. That shown in Fig. 3 is known as a-series type regulator, because of the series disposition of the variable impedance'between the source and the load, and that shownpin Fig. 4 is known as the shunt type. These networks are reproduced here for the reason-that the doublecontrol regulators of the invention shown inthe V subsequent figures may be regarded as being composed of combinations of such structures coupled together in particular mannersto obtain thedesired regulation characteristics. Reference to these networks is useful in determining the signs of the coefiicients G1 and G2 for the design of two-control regulators; The rules for the deterinination'of the signs are arrived at as follows: The impedance relationship required in the circuit of Fig. 3 to permit Equation 2 to be satis fled is aaaia fl a on which requires that a be greater thanunity. The

corresponding relationship for Fig. 4 is same kind, that is, both series type or both shunt type, Kshould be less than unity. If the individ-- from which it is seen that G must be negative for a series: typev regulator and must be positive for. a shunt type regulator. Accordingly, for the dou-i ble control regulators of the invention, the metors G1 and G1 will be negative when the individ-'- ual' regulators to which they correspond are oi the series type and will be positive when the in-. dividual regulators are of the shunt type:

' Whether the *factorK should be greater or less than unity may be determined by the following rules, which have been arrived at empirically from a consideration of -a large number of circuits. -If.both individual regulators are of the ual regulators are of different kinds, one series and the other shunt, K should be greater than unity. The character of the individual regulators is ascertained-by replacing each of the variable impedances in turn by a resistance equal to its reference value and noting the configurations-of the resulting circuit. Another criterion for the determination of the types of the individualregu- A lator is the behaviour of the insertion loss as the variable impedance changes between the extreme values of zero and infinity. For a series regulator the loss increases with the impedance, while for a shunt regulator the loss decreases as the impedance increases.

Network configuration One form oi two-control regulator in accordance with the inventionis shown in Fig. 5. This regulator is suitable for use between a high resistance source R3 and a load of negligibly small impedance, represented by a short circuit between terminals 4, 4'. The variable impedances are provided by fixed constant resistance networks N1 and N2 terminated by variable resistances R1 and R2. This type of variableimpedance is employed in all of the examples illustrated. Theregulator is of the series-series type; consequently, the design factors G1 and G; will both be negative and the factor K will be less than unity.

The configuration is such that coupling between the input terminals 3, 3' and the outputterminals 4, 4' is efi'ected in two ways, first, directly through a shunt coupling resistance Rs and, second, indirectly through the coupled meshes containing the variable impedances. The coupling resistance Rm which constitutes the direct mutual impedance between these meshes plays an important part' in the reduction oi the interaction efiect. Without this coupling the characteristic expressed by Equation 4 could be realized only approximately and reduction of the interaction would require such low values of the resistance R's as would result in excessive flat loss. The circuit comprises seven finite impedances in independent branches; consequently, the six equations referred to in the previous section suflice. for the. determination of six of these in terms of the seventh. Two of the impedances are the reference resistive values of Z1 and Z2 which will be designated respectively .by resistance Rm and some ,parto'i Ba Bindj oharactetietia mesa-"see as -Rue menu. :The wines-or the elements in terms of the sourcedmpedance :Rrare .as-rollows:

all:hiealstransformer;. In the :iiguneu rdenotes the trans-tormeriratio the tvaiue oftwhichrisgiven of the: impedances: o=theorigina1ule=nete work. :Ihearesulting regulator; networkns shown in Fig. 7, the ideal transformer T being replaced hymniaotuahtransformer designed itdsihalfi z'hi gh emoienowinzthez operating drequencyirangea :The winflin saof ithe'transformershould; of cour'se;.':he poledwtomavoid .-a .=.ourrent..re zersa1-.-inthe..output:

As the 5. result .of=r the operations ithe sourcernns. Defiance 15?.feducedjto@iD-EWLCIQWGZZWEJHGSRCMEDG a load impedance of value sia df betwe nitheildad terminals f h'ggn tg work11s .ptherwisjepunchanseu and its egulating charaeterisltic s e nqti afic e i e ".AnQth mo ifi d form may be o tai edrrom heioircuit of Fi 1 y japp y neth sam time oftransformatiieh" first to th -netwonero ed ai t the combination of R with'the resistan'' trans posed by the first.operatiomintmthebranch containing network *Nzl' CB3 proper choice of the fi imtion qf used :in: the. first itransformatiomit i -rqssible-zt qmakeuthe zcombined itransforr-nas thin th L wozsueo ssive steps eequalwitoistheiiratio of tre lfifi tm zfiainvwhiohcase simple vehaneeswf @131 "12 31 91 -:imll lflncg=lfivels :permitmalhmfilthe mnsfo ne stp ib -ael minated- EI'he esult n ne wo ireeeo ealltransformers rshownx: in Fig. 8 v

Re entionsthen regulation A further modified form of Q tips-;- of, the: transformers introduced shyw di sinsth oase=of;i?ig.xfl,.the

application of the principle of duality described R;freeman)Fram r)t eyinte ment OW- anemia cassette: na'works 19. 5,, volume II page 246 Ihi network-,owhichis the exarrjinlet h. zid hi hi i f- 30 m .ue led tt h tance ,3; "d the'yariable' ouplddifectly by the in, "Theistructure may R's' a'n'd the common new n "edits", withl their inputens theirnutm fi "'mi l 'sfi on ed d in I i E 'TIIIiiS th I seo t 'e q 50f whi h .h lqo fi qnfiii h 'h s i ie Qutm t e min j n ec e 21., .Ki heilessth n'uni t e .e w rk' 1 1 1 ren o tha sh wn in Equations iz-f ma tbe used oiii' es en u p renamer new: t elf t-han r f; lq e qorr spond ne ondu an rat o s th act r evens 1E2: re? rep1aced-by their'reciprocals;

The type of regulatorishown in Fig. 9 may be modified asshQwhihQFigI I'O xfc'Si" finite output impedances b y transformations and 8. In thiskase the procedure is a transfer of part of the source admittance to the output terminals and the starting point is the dividing ofilR i i rifigi; into two LparaI l'eI -porti nS, one oi which is combined with Re I to form an L network.

Fig. 11 shows another form of regulator ac cording to the invention'in which one of the com.- ponen re a or 1-i h'= s rie'a ye and th Qthll 'ii iofthez shunt. type".-v Coupling between the independent meshes containing the two variable impedances' ..zi si eifected by the mutual impedance Rm as in:the case of Fig. 5. The network has seven independent impedanc'es include ing the load resistance; consequently, the six de ne a ion diseuS dt-i e h re e i s 15 tionsuflice'fordts comn1ete determination. The

bepderived from the network or'Fig; 5 by the M 0 ml. 1% 'Wo" transmission his cjqristi ted by resistance d n requires ,the 'factors..G1

v of the same kind as were deserib ed-.in 'jeonnectiorn with Figs. 5

explicit formulae for the impedance branches are the following:

life Fem-VT) T1 21" ra l 1 am.)

where F0, F1 and F2 have the values given in Equations 25, subject to the conditions that G1 is negative, G2 is positive, and K is greater than unity.

The dual of the network in Fig. 11 is shown in Fig. 12. Like Fig. 11, this network is of the shunt-series type. The design conditions are that G1 should be positive, G2 negative, and K greater than unity. Equations 27 may be used to determine the element proportions by replacing the resistance ratios or the left-hand sides by the corresponding conductance ratios and replacing F1 and F2 by their reciprocals.

The networks shown in Figs. 5, 9, 11 and 12 may be regarded as prototype networks, since these have basis configurations which admit of complete design computation from the six essential design conditions, namely, the equalities of Equations 3, and the assignment of values to the factors G1, G2 and K. This, of course, amounts to the assignment of numerical values to the six a and b coefiicients in Equation 1. As has been shown, various other forms may be derived fromthe prototypes by the application of well-known network transformations.

Two additional prototype networks, which are related as duals of each other, are shown in Figs.

13 and 14. These are somewhat similar to the networks of Figs. 5 and 9, respectively, but are characterized by negative resistance couplings between the variable impedances. Network 13 is of the series-series type and is adapted for use 50 with a zero load impedance. Network I4 is of the shunt-shunt type and is adapted for use with an infinite load impedance. The formulae for the branch impedances in Fig. 13 are the following:

subject to .theconditions that G1 :iand: G2 fare both negative andK-isless than unity; iASPl'eQ- viously explained, the same formulae may be used to calculate the branch admittances inrFigl, G1 and G2 being both positive in this case.

Physical networks corresponding to Figs. .13 and 14 may be obtained by replacing the T and 1r networks constituted by Ra, Rb and Rm'with equivalent combinations employing positive resistances and'phase reversing transformers, the combination ineach case comprising two series resistances coupled by a transformerhaving'a resistance shunted across one of its windings. In

addition to this, the transformations illustrated in Fig. 6 may be applied as in the casepf Figs. 5 and 9 to transfer some of the source impedance to the load terminals, thereby adapting thecir cuits 'to' finite terminations. The transformed networks are shown in Figs. 15 and 16., llf he transformers have to provide only a small degree of coupling; consequently, they do not have to be of particularly highefficlency. Proper attention to the poling of the windings is necessary to ensure the phase reversal effect predicated bythe negative resistances in the prototype'networks.

When thenetworks of the invention-are employed for the: automatic regulation of a tele' phone line, the variable resistances R'iand R2 may be constituted by thermistors as shown in United States Patent 2,246,307 to H. K; 'Krist to which reference has already been made: Each thermistor may bevaried by means of a heating current under. the control of a'separate pilot-cur rent of suitable frequency. In this wa'y the" 'a'clvantages ofregulatio'n at two frequencies inithe operating-range are substantially achieved,'the two pilot frequencies being chosen to correspond to the maxima of the attenuation components they control.

If desired, theregulators of the invention may be used. to control a'single attenuationvariation, that is, both of the component regulators maybe made to produce the same attenuation variation: In'thiswaythe extent of thevariation may be substantially twice that obtainable with asingle control regulator. f" 1- I s a:

What is claimed is:

1. An attenuation equalizer network comprising at least three independent coupled meshes providing at least three degrees of freedom, two adjustable impedances having frequency dependent characteristics included in separate branches of said network, a signal source andga load, 1111-; cluded in two other branches of said, network, the impedances of the several branches of'said network being soproportioned with respect to each other that the insertion, lossfor transmis-v sion between the said source and said lead is expressed by theequation" pa +a i 2+$12 2 1 wherein a1, a2 and'ao are coefiicients dependent onlyio'n the fixed impedances of the network and are subject to the condition that at is notequal to the product mm, c is the base of the Napierian' logarithms, mi and x2 are the ratios of the values of the adjustable impedances to assigned normal values, 0 is the insertion loss of the system and 00 is the value of 0 when the variable impedances are adjusted to their respective normal values.

2. A network in accordance with claimlin which the network branch impedances exclusive of the adjustable impedances are all substantially pure resistances.- H

3. A network in accordance with claim 1 in which the network branch impedances exclusive of the adjustable impedances are all substantially pure resistances and in which the said adjustable impedances are substantially pure'resist- 5 ances at the adjustments'corresponding to their assigned normal values.

4. A network in accordance with claim 1 in which the source and the load are coupled directly by one branch impedance and'in which the i0 variable impedances are directly coupled by a second branch impedance.

5. "A network in accordance with claim 1 comprising separatevtransmission paths connecting said source and said load, a single resistance element in one of said paths providing direct coupling between the source and load, and the other of said paths comprising a plurality of coupled meshes including the said variable impedances.

SIDNEY DARLINGTON. 

